Electrolytic silver recovery from used photographic solutions is a common way to extend the life of such solutions. An apparatus is known from German patent specification DE 4007906-C1 (Kodak AG) for the electrolytic recovery of silver from solutions containing silver, the apparatus comprising an electrolytic cell, an anode and a removable cathode positioned within the cell and encircling the anode and electrical connectors outside the cell for the cathode and the anode. In this apparatus the cell includes an electrically conductive contact surface which defines an aperture in the side of the cell. The cathode is in the form of a graphite foil carried on a backing of polymeric material. Clamping means in the form of a bolt pass through the aperture and through a hole in the cathode to clamp the cathode against the contact surface to complete an electrical connection from the cathode to the cathode connector.
In an electrolytic cell, the cathode electrical connection is of vital importance since the electrode is the basis for the cathodic reactions which are most important in a de-silvering apparatus. The electrical connection should make sure that enough current can flow to the cathode, without causing a significant ohmic voltage drop between the cathode connector and the cathode itself. The reason for this is that the controlling of the current is usually done by means of the potential on this cathode as one pole and a reference electrode or the anode as the other pole. The potential on the cathode is in the range of some hundred millivolts and should be measured with fair accuracy.
For example, where an Ag/AgCl reference electrode is used, the potential between the cathode and the reference electrode is about 400 mV. When the unit is to perform optimally, meaning employing the maximum current without causing side reactions to occur, the potential should be measured with an accuracy of some millivolts. As a result any ohmic resistance between the cathode and the electrical current supply will cause a measurement fault. While the arrangement disclosed in DE 4007906 may provide a secure electrical connection between the cathode and its exterior connector, the contact surface is small and in use lies below the level of the electrolyte in the cell. This may lead to leakage if the bolt is not tightened sufficiently or to damage to the cathode if it is over-tightened. In any case, contact between the contact surface and the electrolyte may result in corrosion of the contact surface and ultimate build-up of the electrical resistance there across.
A further electrolytic recovery apparatus with removable cathode is disclosed in U.S. Pat. No. 3,985,634. In this apparatus the electrical contact with the cathode is established through a conductor which enters with a lip in sliding contact with the upper edge of a cylindrical cathode as the lid of the apparatus is clamped on the housing. Reliable electrical contact is obtained by a tab and corresponding screw that apply extra pressure to the point where the lip and cathode edge contact. This construction has the same disadvantage as the apparatus described hereinbefore, namely an electrical contact with limited contact surface. Also the protruding tab of the lid is vulnerable and the presence of an electrical connection on the lid makes its manipulation less convenient .
Still another recovery apparatus is disclosed in CH-A-647,005. Electrical contact with a cylindrical removable cathode occurs through elastic fingers protruding from the lid of the apparatus. The contact surface is limited and deformation of the contact fingers by careless manipulation of the lid may impede the successful use on the long run.
It is an object of the present invention to provide an electrolytic cell in which a secure electrical connection can be assured between the cathode and its exterior connector over a large surface and without the risk of leakage or the build-up of resistive corrosion.